It's not 6 missions, it's 6 maps. You've got the story missions, plus contracts that give the game endless replayability, elusive targets, and maybe bonus missions on the same maps. It's not a game where you just do every mission once and you're done with it, part of the fun is replaying them with more knowledge to make perfect runs, try out new ways to kill the targets, complete challenges that unlock new options for the map...